He definitely didn't use it voluntarily, and he likely didn't use it at all.
Separately, Niji has stealth abilities. There is no coincidence that he said it and then disappeared. Stealth black is unlike the other Raid Suits, so it's less likely to represent his Germa power and more likely to be an invention based on general Raid Suit abilities or inspired by the Germa comic. The translation I read of the O Soba Mask vivre card mentioned nothing about the power being exclusive.
Also
Fast forward to maybe the eighth minute. This isn't canon, but there is the fact that apparently the game has Niji disappear with stealth and require you to use CoO to get him.
And more clarity in my response to you
I showed you how this is wrong.